How to get face normal from a mesh loop?
Loop the faces
Suggest this is easier to loop the faces and get the loop indices.
for f in mesh.polygons:
    print(f.index, f.loop_indices)

default cube
0 range(0, 4)
1 range(4, 8)
2 range(8, 12)
3 range(12, 16)
4 range(16, 20)
5 range(20, 24)